Surface-normal optical path structure for infrared photodetection

ABSTRACT

A SiGe surface-normal optical path photodetector structure and a method for forming the SiGe optical path normal structure are provided. The method comprises: forming a Si substrate with a surface; forming a Si feature, normal with respect to the Si substrate surface, such as a via, trench, or pillar; depositing SiGe overlying the Si normal feature to a thickness in the range of 5 to 1000 nanometers (nm); and, forming a SiGe optical path normal structure having an optical path length in the range of 0.1 to 10 microns. Typically, the SiGe has a Ge concentration in the range from 5 to 100%. The Ge concentration may be graded to increase with respect to the deposition thickness. For example, the SiGe may have a 20% concentration of Ge at the Si substrate interface, a 30% concentration of Ge at a SiGe film top surface, and a thickness of 400 nm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to integrated circuit (IC) fabricationprocesses and, more particularly, to a surface-normal infrared opticalpath structure and corresponding fabrication method.

2. Description of the Related Art

There are many applications for photodetection in the near infraredregion (the wavelength between 0.7 micron to 2 microns), such as infiber-optical communication, security, and thermal imaging. AlthoughIII-V compound semiconductors provide superior optical performance overtheir silicon (Si)-based counterparts, the use of Si is desirable, asthe compatibility of Si-based materials with conventional Si-ICtechnology promises the possibility of cheap, small, and highlyintegrated optical systems.

Silicon photodiodes are widely used as photodetectors in the visiblelight wavelengths due to their low dark current and the above-mentionedcompatibility with Si IC technologies. Further, silicon-germanium(Si_(1-x)Ge_(x)) permits the photodetection of light in the 0.8 to 1.6micron wavelength region.

However, the SiGe alloy has larger lattice constant than the Si lattice,so film thickness is a critical variable in the epitaxial growth of SiGeon Si substrates. While a thick SiGe is desirable for light absorption,too thick of a SiGe film causes a defect generation that is responsiblefor dark currents. This critical SiGe thickness is dependent upon the Geconcentration and device process temperature. Higher Ge concentrationsand higher device process temperatures result in the formation ofthinner SiGe film thicknesses. In common practice, the SiGe criticalthickness is in the range of a few hundred angstroms, to maximum of afew thousand angstroms. Once the SiGe thickness is grown beyond itscritical thickness, lattice defects in SiGe are inevitable. As mentionedabove, an IR photo detector built from a SiGe film with lattice defectsgenerates large dark currents and noise.

Quantum efficiency is a measure of the number of electron-hole pairsgenerated per incident photon, and it is a parameter for photodetectorsensitivity. Quantum efficiency is defined as:η=(I _(p) /q)/(P _(opt) /hν)

-   -   where I_(p) is the current generated by the absorption of        incident optical power P_(opt) at the light frequency v.

FIG. 1 is a graph showing the relationship between quantum efficiencyand the percentage of Ge in a SiGe film. One of the key factors indetermining quantum efficiency is the absorption coefficient, a. Siliconhas a cutoff wavelength of about 1.1 microns and is transparent in thewavelength region between 1.3 to 1.6 microns. The SiGe absorption edgeshifts to the red with an increasing Ge mole fraction and is shown inFIG. 1. The absorption coefficient of any SiGe alloy is relatively smalland the limited thickness dictated by the critical thickness furtherlimits the ability of SiGe films to absorb photons.

As noted above, the major goals of SiGe-based photodetection are highquantum efficiency and the integration of these SiGe photodetectors withthe existing Si electronics. One way to increase the optical path, andimprove the quantum efficiency, is to form the optical path in the sameplane as the SiGe film, along the substrate surface in which the SiGe isdeposited. Thus, light propagates parallel to the heterojunction(SiGe/Si) interface. However, this optical path design necessarilylimits the design of IR detectors.

It would be advantageous if an efficient SiGe IR photodetector could befabricated having an optical path that need not be formed in parallelwith a Si substrate surface.

SUMMARY OF THE INVENTION

The present invention SiGe optical path structure (absorbs IR wavelengthlight that is normal to a silicon substrate surface and parallel to theSiGe/Si heterojunction interface, increasing the length of the opticalpath. Therefore, a two-dimensional IR image detection can be realizedwith a thin SiGe thickness. Because of the relatively poor quantumefficiencies associated with SiGe, the IR absorption length of SiGe mustbe long, and conventionally a thick SiGe layer is needed to absorb highamounts of IR energy. However, it is very difficult to grow defect-freethick SiGe film on Si substrate because of the lattice mismatch betweenthese two materials. The present invention eliminates the need for athick SiGe film. SiGe film is grown on the sidewall of a Si substratetrench or pillar, forming a relatively long optical path for lightnormal to the substrate surface. The present invention's use ofrelatively thin SiGe films permits a SiGe IR photodetector to be easilyintegrated with Si CMOS devices, with minimal lattice mismatch.

Accordingly, a method is provided for forming a SiGe optical pathstructure, normal to a Si substrate surface, for the purpose of IRphotodetection. The method comprises: forming a Si substrate with asurface; forming a Si feature, normal with respect to the Si substratesurface, such as a via, trench, or pillar; depositing SiGe overlying theSi normal feature to a thickness in the range of 5 to 1000 nanometers(nm); and, forming a SiGe optical path normal structure having anoptical path length in the range of 0.1 to 10 microns.

In some aspects of the method, depositing SiGe overlying the Si normalfeature includes depositing SiGe with a Ge concentration in the rangefrom 5 to 100%. In other aspects, the SiGe is deposited with a graded Geconcentration that increases with respect to the deposition thickness.For example, the SiGe may have a 20% concentration of Ge at the Sisubstrate interface, a 30% concentration of Ge at a SiGe film topsurface, and a thickness of 400 nm.

Additional details of the above-described method and a SiGe optical pathstructure, normal to a Si substrate surface, are provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between quantum efficiencyand the percentage of Ge in a SiGe film.

FIG. 2 is a cross-sectional view of the present invention SiGe opticalpath structure, normal to a Si substrate surface, for IR photodetection.

FIG. 3 is a cross-sectional view of an alternate aspect of the SiGeoptical path structure of FIG. 2.

FIG. 4 is a plan view of the present invention optical path structure.

FIG. 5 is a cross-sectional view of the present invention IRphotodetector.

FIG. 6 is a cross-sectional view of FIG. 2, featuring an alternateaspect of the invention.

FIG. 7 is a cross-sectional view of a preliminary step in the formationof a PIN diode SiGe IR photodetector using a trench surface-normalfeature.

FIG. 8 is a cross-sectional view of the photodetector of FIG. 7following a photoresist process to form trenches in the Si substrate(N-well).

FIG. 9 is a cross-sectional view of the photodetector of FIG. 8following the epitaxial growth of SiGe on photodiode area.

FIG. 10 is a cross-sectional view of the photodetector of FIG. 9following a photoresist and etching of the P+Si and SiGe layers.

FIG. 11 is a cross-sectional view of the photodetector of FIG. 10following an ILD deposition and the formation of interlevel contact tothe CMOS transistors and the present invention IR photodiode.

FIG. 12 is a cross-sectional view of SiGe optical path structure of FIG.6 with the addition of a microlens.

FIG. 13 is a cross-sectional view of the photodetector of FIG. 11 withthe addition of a microlens.

FIG. 14 is a cross-sectional view of a Schottky diode IR photodetectorusing a surface-normal SiGe optical path.

FIG. 15 is a cross-sectional view of an npn bipolar IR detector using asurface-normal SiGe optical path

FIG. 17 is a flowchart illustrating the present invention method forforming a SiGe optical path structure, normal to a Si substrate surface,for IR photodetection.

FIG. 18 is a flowchart illustrating the present invention method forforming an IR photodetector with a SiGe optical path structure, normalto a Si substrate surface.

FIG. 16 is a flowchart illustrating the present invention method forphotodetecting IR energy using a SiGe surface-normal optical pathstructure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a cross-sectional view of the present invention SiGe opticalpath structure, normal to a Si substrate surface, for IR photodetection.The structure 200 comprises a Si substrate 202 with a surface 204. A Sifeature 206 is normal with respect to the Si substrate surface 204. Asshown, the feature 206 can be a via 206 a, a trench 206 b, or a pillar206 c. A surface-normal SiGe optical path 208, shown with doublecross-hatched lines, overlies the Si feature 206.

The Si substrate surface (interface) 204 is formed in a first plane 210parallel to the substrate surface 204. SiGe is epitaxially grown on theSi surface 204 and Si feature 206. The surface-normal SiGe optical path208 is formed in a second plane 212, normal to the first plane 210. Thatis, the optical path 208 is normal to the substrate surface 204.Alternately stated, the feature 206 has an element or structure in avertical plane that is perpendicular to the horizontal surface 204.Note, that the feature 206 may also include an element or structure, atrench bottom or pillar top for example, that is in a plane parallel tothe first plane 210.

The optical path 208 has a thickness 214 in the range of 5 to 1000nanometers (nm). The surface-normal SiGe optical path 208 has an opticalpath length 216 in the range of 0.1 to 10 microns, in the second plane212.

In some aspects, the surface-normal SiGe optical path 208 includes a Geconcentration in the range from 5 to 100%. In other aspects, thesurface-normal SiGe optical path 208 includes graded Ge concentrationthat increases with respect to the deposition thickness. For example,the surface-normal SiGe optical path 208 may have a 20% concentration ofGe at the Si substrate interface 204, a 30% concentration of Ge at aSiGe film top surface 220, and a thickness 214 of 400 nm.

FIG. 3 is a cross-sectional view of an alternate aspect of the SiGeoptical path structure of FIG. 2. As in FIG. 2, a Si substrate 202 has asurface 204. The structure 200 further comprises at least one Si layer300 overlying SiGe 302, so that the surface-normal SiGe optical path 208includes a plurality of SiGe layers overlying Si. In this example, a via206 a is shown, with a SiGe first layer 302 and a second SiGe layer 304.Although an optical path 208 is shown with two SiGe layers (302/304) anda single interposing Si layer 300, the present invention is not limitedto any particular number of SiGe/Si interfaces or layers. Further, thefinal SiGe layer (304 in this example) may fill the via 206 a. Neitheris the multilayer optical path structure limited to just a viasurface-normal feature.

FIG. 4 is a plan view of the present invention optical path structure.Viewing both FIGS. 2 and 4, when the Si feature is a trench 206 b, thereare a pair of sidewalls 400 a and 400 b. In one aspect, thesurface-normal SiGe optical path 208 is an optical path pair-structureadjacent the trench sidewalls 400 a and 400 b (see FIG. 4). Of course,the trench 206 b typically has ends (not shown), which would constitutea second pair of sidewalls. Then, the optical path 208 mightadditionally include optical paths adjacent these trench-end sidewalls.In a different aspect, the surface-normal SiGe optical path 208 is auni-structure that fills the trench 206 b (see FIG. 2).

Returning to FIG. 4, when the Si feature is a pillar 206 c, the pillar206 c has two pairs of sidewalls, a first pair 402 a and 402 b and asecond pair 404 a and 404 b. The surface-normal SiGe optical path 208 isan optical path array-structure adjacent the corresponding pillarsidewall pairs 402 a/402 b and 404 a/404 b. In other aspects (not shown)the optical path structure is composed of SiGe layers adjacent a subsetof the pillar sidewalls. Likewise, an optical path may be formedadjacent a pillar with rounded sidewalls.

In other aspects, the Si normal feature is via 206 a with two pairs ofsidewalls, a first pair of sidewalls 410 a and 410 b, and a second setof sidewalls 412 a and 412 b. Then, the surface-normal SiGe optical path208 is an optical path array-structure adjacent the corresponding trenchsidewall pairs 410 a/410 b and 412 a/412 b. Alternately, thesurface-normal SiGe optical path is a uni-structure that fills the via(see FIG. 2). Likewise, an optical path may be formed adjacent a viawith rounded sidewalls.

FIG. 5 is a cross-sectional view of the present invention IRphotodetector. The photodetector 500 comprises an interconnect 502 inelectrical communication with a CMOS active region (not shown) formed ina Si substrate 504. The active region can be transistor source, drain,gate, or a diode region. The Si substrate 504 has a surface (interface)505. A Si feature 506 is shown normal with respect to the Si substratesurface 505 in electrical communication with the interconnect 502. Inthis example, the feature 506 is a via. However, in other aspects thefeature can be a trench or a pillar. A surface-normal SiGe optical path508 overlies the Si feature 506.

In some aspects, an interlayer dielectric 510, such as SiO2, overliesthe surface-normal SiGe optical path 508, and a microlens 512 overliesthe interlayer dielectric 510 in optical communication with thesurface-normal SiGe optical path 508. Additional details of the SiGeoptical path are presented above in the explanations of FIGS. 2-4.Examples of CMOS active regions follow.

Functional Description

FIG. 6 is a cross-sectional view of FIG. 2, featuring an alternateaspect of the invention. The present invention optical structure iscreated normal to a Si substrate surface. This can be accomplished usingstandard Si IC trench, pillar, or hole (via) processes. A SiGe (Geconcentration 5% to 100%) is epitaxially deposited on the Si. Two simplestructures, a Si trench and Si pillar are shown. The SiGe is epitaxiallydeposited on Si to a thickness that is less than the critical thickness,so that no defects are generated. As an alternative to SiGe depositionwith fixed concentration of Ge, a graded SiGe layer can be deposited.Another alternative is to form a quantum well SiGe structure thatincludes multiple Si and SiGe layers (SiGe/Si/SiGe/Si . . . ). SiGe canbe used to either fill the trench or line the trench sidewalls

FIG. 7 is a cross-sectional view of a preliminary step in the formationof a PIN diode SiGe IR photodetector using a trench surface-normalfeature. This invention can be incorporated with various device types tofabricate high efficient IR photodetectors. These devices include, butare not limited to, PN diodes, PIN type diodes, heterojunctionphototransistors, quantum well photodiodes, and Schottky diodes.Standard CMOS devices can be integrated with the IR detectors on asingle Si wafer. As with any conventional CMOS procedure, an interleveldielectric (ILD) deposition is performed. An N-well can be used as then-layer of the PIN diode, or additional processes can be performed toform an n-layer.

FIG. 8 is a cross-sectional view of the photodetector of FIG. 7following a photoresist process to form trenches in the Si substrate(N-well). The trenches have a depth in the range of 0.1 to 10 microns.

FIG. 9 is a cross-sectional view of the photodetector of FIG. 8following the epitaxial growth of SiGe on photodiode area. SiGe isnon-doped to form a SiGe intrinsic layer. Note, a SiGe deposition on topof ILD becomes polycrystalline. The SiGe thickness is 0.05 to 0.5microns. Next, Si is epitaxially grown and doped to be P+. The P+dopingcan be an result of in-situ doping during Si growth, or ion implantationafter Si growth. The P+thickness is in the range of 0.05 to 1 micron.

FIG. 10 is a cross-sectional view of the photodetector of FIG. 9following a photoresist and etching of the P+Si and SiGe layers.

FIG. 11 is a cross-sectional view of the photodetector of FIG. 10following an ILD deposition and the formation of interlevel contact tothe CMOS transistors and the present invention IR photodiode.

FIG. 12 is a cross-sectional view of SiGe optical path structure of FIG.6 with the addition of a microlens.

FIG. 13 is a cross-sectional view of the photodetector of FIG. 11 withthe addition of a microlens. IR detectors with SiGe vertical sidewallshave improved quantum efficiency, but the effective area for the IRdetection (the optical path length) depends on the surface-normalfeature (trench/via/pillar) layout. Another way to improve the areaefficiency is to add a microlens, to focus the incident IR into thetrench, as shown in FIGS. 12 and 13. The IR reflection at the Si/SiGeinterface can also improve the light absorption. Note the reflection oflight at the Si/SiGe interfaces. Also note that in FIG. 13 that eachtrench has its own microlens to focus the light and maximize the IRabsorption in SiGe.

FIG. 14 is a cross-sectional view of a Schottky diode IR photodetectorusing a surface-normal SiGe optical path. The diode can be formed ineither a P-well or an N-well. The metal deposition can be a materialsuch as Pt, Ir, or Pt/Ir.

FIG. 15 is a cross-sectional view of an npn bipolar IR detector using asurface-normal SiGe optical path. As shown, the transistor is formed inan N-well. The SiGe is p-type doped with boron either by in-situ dopingor by ion implantation after deposition. The overlying Si layer isin-situ P-doped or As-doped n-type Si. Alternately, intrinsic Si can beimplanted with dopants of As or P to form n-type Si layer.

FIG. 17 is a flowchart illustrating the present invention method forforming a SiGe optical path structure, normal to a Si substrate surface,for IR photodetection. Although the method (and the method describingFIGS. 18 and 16, below) is depicted as a sequence of numbered steps forclarity, no order should be inferred from the numbering unlessexplicitly stated. It should be understood that some of these steps maybe skipped, performed in parallel, or performed without the requirementof maintaining a strict order of sequence. The method starts at Step1700.

Step 1702 forms a Si substrate with a surface. Step 1704 forms a Sifeature, such as a via, trench, or pillar, normal with respect to the Sisubstrate surface. Note, the invention is not necessarily limited tojust these three example features. Step 1706 deposits SiGe overlying theSi normal feature (and Si substrate surface), to a thickness in therange of 5 to 1000 nanometers (nm). Step 1708 forms a SiGe optical pathnormal structure having an optical path length in the range of 0.1 to 10microns. As used herein, a normal structure is intended to describe a Sisubstrate surface-normal structure. Alternately expressed, thesurface-normal features have a length of 0.1 to 10 microns.

Typically, Step 1706 deposits SiGe with a Ge concentration in the rangefrom 5 to 100%. In some aspects, SiGe is deposited with a graded Geconcentration that increases with respect to the deposition thickness.For example, the SiGe may have a 20% concentration of Ge at the Sisubstrate interface, a 30% concentration of Ge at a SiGe film topsurface, and a thickness of 400 nm.

Other aspects of the method include additional steps. Step 1707 adeposits a Si layer overlying the SiGe. Step 1707 b deposits SiGeoverlying the Si layer. Then, forming a SiGe normal optical pathstructure in Step 1708 includes forming a normal optical path structurewith a plurality of SiGe layers. Note, Steps 1707 a and 1707 b may beiterated a number of times to build up a plurality of SiGe/Si layers.

For example, if Step 1704 forms a trench with a pair of sidewalls, Step1706 may deposit SiGe sidewalls overlying the trench sidewalls. Then,Step 1708 forms a SiGe optical path pair-structure. Alternately, Step1706 fills the trench with SiGe and Step 1708 forms a SiGe optical pathuni-structure.

In another example, Step 1704 forms a pillar with two pairs of sidewallsand Step 1706 deposits SiGe sidewalls overlying the two pairs of pillarsidewalls. Then, Step 1708 forms an optical path array-structureadjacent the corresponding pillar sidewall pairs. Alternately, the SiGeoptical path structure can be formed on a subset of the four pillarsidewalls.

In another example, Step 1704 forms a via with two pairs of sidewallsand Step 1706 deposits SiGe sidewalls overlying the two pairs of viasidewalls. Then, Step 1708 forms an optical path array-structureadjacent the corresponding via sidewall pairs. As above, the opticalpath structure can be formed on a subset of the via sidewalls.Alternately, Step 1706 fills the via with SiGe and Step 1708 forms anoptical path uni-structure.

Other aspects of the method include further steps. Step 1710 forms aninterlayer dielectric overlying the SiGe optical path normal structure.Step 1712 forms a microlens overlying the interlayer dielectric inoptical communication with the SiGe optical path normal structure.

FIG. 18 is a flowchart illustrating the present invention method forforming an IR photodetector with a SiGe optical path structure, normalto a Si substrate surface. The method starts at Step 1800. Step 1802forms a Si substrate with a surface. Step 1804 forms an interconnect inelectrical communication with a CMOS active region such as a source,drain, gate, or a diode region. Examples of such active regions arepresented in FIGS. 5 through 15. Step 1806 forms a Si feature, normalwith respect to the Si substrate surface. Step 1808 deposits SiGeoverlying the Si normal feature. Step 1810 forms a SiGe optical pathnormal structure in electrical communication with the CMOS activeregion, through the interconnect. Step 1812 forms an interlayerdielectric overlying the SiGe optical path normal structure. Step 1814forms a microlens overlying the interlayer dielectric in opticalcommunication with the SiGe optical path normal structure. Details ofthe SiGe optical path structure are presented in the explanation of FIG.17, above.

FIG. 16 is a flowchart illustrating the present invention method forphotodetecting IR energy using a SiGe surface-normal optical pathstructure. The method starts at Step 1900. Step 1902 accepts IR photonshaving a trajectory normal to a Si substrate surface. Step 1904 absorbsthe IR photons through a SiGe surface-normal optical path structure.Step 1906 generates a current in response to absorbing the IR photons.Step 1908 conducts the current into a CMOS active region.

In some aspects, accepting IR photons having a trajectory normal to a Sisubstrate surface in Step 1902 includes accepting IR photons having awavelength in the range of 0.8 to 1.6 microns.

In other aspects, absorbing the IR photons through a SiGe surface-normaloptical path structure in Step 1904 includes absorbing 1.1 micronwavelength IR photons with an efficiency of approximately 7%, responsiveto an optical path structure length of 10 microns. In a different aspectStep 1904 absorbs 1.1 micron wavelength IR photons with an efficiency inthe range of 0.07 to 7% efficiency, responsive to an optical pathstructure length in the range of 0.1 to 10 microns.

A surface-normal SiGe optical path structure and correspondingfabrication process have been presented. Simple surface-normal featuressuch as vias, trenches, and pillars have been used to illustrate theinvention. However, the invention may also be applied to morecomplicated features. Likewise, although SiGe films have been described,the invention is not necessarily limited to a particular light-absorbingfilm or a particular wavelength of light. Other variations andembodiments of the invention will occur to those skilled in the art.

1. A method for forming a silicon-germanium (SiGe) optical pathstructure, normal to a silicon (Si) substrate surface, for infrared (IR)photodetection, the method comprising: forming a Si substrate with asurface; forming a Si feature, normal with respect to the Si substratesurface; depositing SiGe overlying the Si normal feature; and, forming aSiGe optical path normal structure.
 2. The method of claim 1 whereinforming a Si feature, normal with respect to the Si substrate surfaceincludes forming a feature selected from the group including a via,trench, and pillar.
 3. The method of claim 1 wherein depositing SiGeoverlying the Si normal feature includes depositing SiGe to a thicknessin the range of 5 to 1000 nanometers (nm).
 4. The method of claim 1wherein forming a SiGe optical path normal structure includes forming aSiGe normal structure having an optical path length in the range of 0.1to 10 microns.
 5. The method of claim 1 wherein depositing SiGeoverlying the Si normal feature includes depositing SiGe with a Geconcentration in the range from 5 to 100%.
 6. The method of claim 1wherein depositing SiGe overlying the Si normal feature includesdepositing SiGe with a graded Ge concentration that increases withrespect to the deposition thickness.
 7. The method of claim 6 whereinthe SiGe has a 20% concentration of Ge at the Si substrate interface, a30% concentration of Ge at a SiGe film top surface, and a thickness of400 nm.
 8. The method of claim 1 further comprising: depositing a Silayer overlying the SiGe; depositing SiGe overlying the Si layer; and,wherein forming a SiGe normal optical path structure includes forming anormal optical path structure with a plurality of SiGe layers.
 9. Themethod of claim 1 wherein forming a Si feature, normal with respect tothe Si substrate surface includes forming a trench with a pair ofsidewalls; wherein depositing SiGe overlying the Si normal featureincludes depositing SiGe sidewalls overlying the trench sidewalls; and,wherein forming a SiGe optical path normal structure includes forming anoptical path pair-structure.
 10. The method of claim 1 wherein forming aSi feature, normal with respect to the Si substrate surface includesforming a trench; wherein depositing SiGe overlying the Si normalfeature includes filling the trench with SiGe; and, wherein forming aSiGe optical path normal structure includes forming an optical pathuni-structure.
 11. The method of claim 1 wherein forming a Si feature,normal with respect to the Si substrate surface includes forming apillar with two pairs of sidewalls; wherein depositing SiGe overlyingthe Si normal feature includes depositing SiGe sidewalls overlying thetwo pairs of pillar sidewalls; and, wherein forming a SiGe optical pathnormal structure includes forming an optical path array-structureadjacent the corresponding pillar sidewall pairs.
 12. The method ofclaim 1 wherein forming a Si feature, normal with respect to the Sisubstrate surface includes forming a via with two pairs of sidewalls;wherein depositing SiGe overlying the Si normal feature includesdepositing SiGe sidewalls overlying the two pairs of via sidewalls; and,wherein forming a SiGe optical path normal structure includes forming anoptical path array-structure adjacent the corresponding via sidewallpairs.
 13. The method of claim 1 wherein forming a Si feature, normalwith respect to the Si substrate surface includes forming a via; whereindepositing SiGe overlying the Si normal feature includes filling the viawith SiGe; and, wherein forming a SiGe optical path normal structureincludes forming an optical path uni-structure.
 14. The method of claim1 further comprising: forming an interlayer dielectric overlying theSiGe optical path normal structure; and, forming a microlens overlyingthe interlayer dielectric in optical communication with the SiGe opticalpath normal structure.
 15. A method for forming an infrared (IR)photodetector with a silicon-germanium (SiGe) optical path structure,normal to a silicon (Si) substrate surface, the method comprising:forming a Si substrate with a surface; forming an interconnect inelectrical communication with a CMOS active region selected from thegroup including a source, drain, gate, and a diode region; forming a Sifeature, normal with respect to the Si substrate surface; depositingSiGe overlying the Si normal feature; and, forming a SiGe optical pathnormal structure in electrical communication with the active region,through the interconnect.
 16. The method of claim 15 further comprising:forming an interlayer dielectric overlying the SiGe optical path normalstructure; and, forming a microlens overlying the interlayer dielectricin optical communication with the SiGe optical path normal structure.17. A method for photodetecting infrared (IR) energy using asilicon-germanium (SiGe) surface-normal optical path structure, themethod comprising: accepting IR photons having a trajectory normal to asilicon (Si) substrate surface; absorbing the IR photons through a SiGesurface-normal optical path structure; generating a current in responseto absorbing the IR photons; and, conducting the current into a CMOSactive region.
 18. The method of claim 17 wherein accepting IR photonshaving a trajectory normal to a Si substrate surface includes acceptingIR photons having a wavelength in the range of 0.8 to 1.6 microns. 19.The method of claim 17 wherein absorbing the IR photons through a SiGesurface-normal optical path structure includes absorbing 1.1 micronwavelength IR photons with an efficiency of approximately 7%, responsiveto an optical path structure length of 10 microns.
 20. The method ofclaim 17 wherein absorbing the IR photons through a SiGe surface-normaloptical path structure includes absorbing 1.1 micron wavelength IRphotons with an efficiency in the range of 0.07 to 7% efficiency,responsive to an optical path structure length in the range of 0.1 to 10microns.
 21. A silicon-germanium (SiGe) optical path structure, normalto a silicon (Si) substrate surface, for infrared (IR) photodetection,the structure comprising: a Si substrate with a surface; a Si feature,normal with respect to the Si substrate surface; and, a surface-normalSiGe optical path overlying the Si feature.
 22. The structure of claim21 wherein the Si feature is selected from the group including a via,trench, and pillar.
 23. The structure of claim 21 wherein the Sisubstrate surface is formed in a first plane; and, wherein thesurface-normal SiGe optical path is formed in a second plane, normal tothe first plane, with a thickness in the range of 5 to 1000 nanometers(nm).
 24. The structure of claim 21 wherein the surface-normal SiGeoptical path has an optical path length in the range of 0.1 to 10microns, in the second plane.
 25. The structure of claim 21 wherein thesurface-normal SiGe optical path includes a Ge concentration in therange from 5 to 100%.
 26. The structure of claim 21 wherein thesurface-normal SiGe optical path includes graded Ge concentration thatincreases with respect to the deposition thickness.
 27. The structure ofclaim 26 wherein the surface-normal SiGe optical path has a 20%concentration of Ge at the Si substrate interface, a 30% concentrationof Ge at a SiGe film top surface, and a thickness of 400 nm.
 28. Thestructure of claim 21 further comprising: at least one Si layeroverlying SiGe; and, wherein the surface-normal SiGe optical pathincludes a plurality of SiGe layers overlying Si.
 29. The structure ofclaim 21 wherein the Si feature is a trench with a pair of sidewalls;and, wherein surface-normal SiGe optical path is an optical pathpair-structure adjacent the trench sidewalls.
 30. The structure of claim21 wherein the Si feature is a trench; and, wherein the surface-normalSiGe optical path is an optical path uni-structure filling the trench.31. The structure of claim 21 wherein the Si feature is a pillar withtwo pairs of sidewalls; and, wherein the surface-normal SiGe opticalpath is an optical path array-structure adjacent the correspondingpillar sidewall pairs.
 32. The structure of claim 21 wherein the Sinormal feature is a via with two pairs of sidewalls; and, wherein thesurface-normal SiGe optical path is an optical path array-structureadjacent the corresponding via sidewall pairs.
 33. The structure ofclaim 21 wherein the Si normal feature is a via; and, wherein thesurface-normal SiGe optical path is a optical path uni-structure fillingthe via.
 34. An infrared (IR) photodetector comprising: a CMOS activeregion formed in a silicon (Si) substrate with a surface, the activeregion selected from the group including a transistor source, drain,gate, and a diode region; an interconnect in electrical communicationwith the active region; a Si feature, normal with respect to the Sisubstrate surface, and in electrical communication with theinterconnect; and, a surface-normal SiGe optical path overlying the Sifeature.
 35. The photodetector of claim 34 further comprising: aninterlayer dielectric overlying the surface-normal SiGe optical path;and, a microlens overlying the interlayer dielectric in opticalcommunication with the surface-normal SiGe optical path.